Hexamethylenediamine, also abbreviated HMD in the rest of the description, is an important chemical for industrial applications. The great part of the world production of this substance is used for manufacturing nylon 6-6 via condensation with adipic acid, with minor amounts used in the production of polyurethanes or as cross-linking agent in epoxy resins.
The present process for the production of HMD is essentially based on the teachings of U.S. Pat. No. 3,821,305, entirely incorporated herein by reference. Briefly, in the process described in said patent, adiponitrile (also abbreviated ADN in the rest of the description) and hydrogen are simultaneously fed to the bottom of a reaction column, filled with HMD produced in the reaction, a caustic alkali, water and a Raney-type catalyst (e.g., Raney Nickel). Concentration ratios of the species present in the reactor must be kept in given ranges, in order to ensure the total conversion of ADN with a good yield to HMD, and very low amounts of impurities; also important for assuring good results are the continuous agitation of the reacting system, a hydrogen partial pressure kept constantly in the range of 10 to 50 bar (gauge pressure), and a temperature maintained in the range from 60 to 100° C.
Under these conditions, the reaction medium is essentially a liquid mixture HMD-water, containing between 93% and 97% by weight of HMD, in which the particles of the metallic catalyst are suspended; the caustic alkali (preferably caustic soda, NaOH) is essentially insoluble in the HMD-water mixture and gives rise to a separated liquid phase, a water solution of concentration between about 25% to 55% by weight of the alkali, that is present in the reacting system in the form of a film on the surface of the catalyst particles.
A known problem with this type of process is the high rate of deactivation of the catalyst, which leads to a reduced overall rate of ADN conversion and a reduced selectivity towards HMD, thus leading to an increase in the production of impurities.
A first possible cause of deactivation is the settling of the catalyst, which is heavier than the HMD-water mixture, in dead-ends of the reactor vessel; the catalyst that settles in certain points of the vessel does not take part anymore in the reaction and undergoes rapid deactivation, and must be replaced with fresh catalyst.
Another main factor in the catalyst deactivation is the build-up of nitrile groups of ADN on the catalyst surface. The paper “Hydrogenation of adiponitrile catalyzed by Raney Nickel. Use of intrinsic kinetics to measure gas-liquid mass transfer in a gas induced stirred slurry reactor”, C. Mathieu et al, Chemical Engineering Science vol. 47 no. 9-11, 2289-94 (1992), describes that Raney-type catalysts such as nickel or cobalt, when used in low-pressure hydrogenation processes as in the present case, are rapidly deactivated by the accumulation of non-reacted nitrile groups at the liquid-solid interface between the reaction medium and the catalyst. Said accumulation can be favoured by a non-uniform dispersion of ADN in the reacting mixture, that can give rise to zones of the mixture where a high concentration of this chemical is present.
According to U.S. Pat. No. 3,821,305 it is necessary to adopt measures for keeping the catalyst activity above a minimum value. The effectiveness of the “hydrogenating capability” of the catalyst bulk on the reaction can be expressed as a function of the concentration of the suspended catalyst (e.g., by weight) in the reaction medium together with its average residual potential activity in terms of amount of hydrogen contained in the catalyst, measured e.g. in normal cm3 of H2 per gram of catalyst, Ncm3H2/gCAT; typical values of activity of the fresh catalyst are comprised between about 60 and 80 Ncm3H2/gCAT. In practice, only part of this potential capability can be used for the reaction, due to phenomena occurring in the course of the running time, like the catalyst particle size mechanical fragmentation, increasing the fines content of the catalyst's mass, or the catalyst particles poisoning, leading to a statistical distribution of activities in the catalyst bulk, according to different levels of deactivation of the catalyst particles.
A good operation of the hydrogenation on a steady state condition must take care of these phenomena, introducing the operations helpful to maintain the “hydrogenation capability” of the catalyst bulk constant during the operation and well related to the desired production rate.
The paper “Gas holdup and liquid recirculation in gas-lift reactors”, Y. C. Hsu et al., Chemical Engineering Science, Vol. 35, 135-141 (1980) teaches that the presence of zones of the reacting mixture of high ADN concentration can be avoided by adopting conditions that create turbulent flow in the reaction medium; this can be obtained for instance through high liquid recirculation speeds in the reactor. The same problem is tackled by U.S. Pat. No. 6,281,388 B1, that discloses the use of mixers, preferably a static mixer, to enhance the dispersion in the reaction mixture of the ADN fed to the same. Finally, Patent application US 2010/0130789 A1 describes a HMD production process taking place in a plug-flow reactor, in which the catalyst is maintained at the desired activity level through the control of the feeding rate into the reactor of the nitrile and/or the catalyst, in such a way as to keep in a desired range the ratio of moles of nitrile fed per unit time to flow rate by weight of catalyst.
The adoption of these measures, however, can only reduce but not avoid the deactivation of the catalyst, that must thus be continuously refreshed to maintain the efficiency of the reaction at suitable levels for industrial applications. Refreshing of the catalyst is generally obtained in known processes by extracting a portion of the reacting mixture from the reactor; separating the catalyst from the liquid phase, which is sent to purification processes downstream the reaction for the recover of HMD; subjecting the spent (or partially spent) catalyst to a regeneration treatment; and feeding back the regenerated catalyst to the reaction. In practice, in order to improve the average activity of the catalyst, generally only part of regenerated catalyst is fed back into the reactor, the remainder part being discharged; the amount of discharged catalyst is replaced by an equal quantity of fresh catalyst.
The regeneration of the spent (or partially spent) catalyst is aimed to remove organic compounds (generally polyamines) and inorganic compounds (generally alumina and aluminates) formed in the hydrogenation process, that could clog the pores of the catalyst, hindering the transport of hydrogen to the inner surfaces of the pores and thus inhibiting, and in the end quenching, the catalyst activity.
The regeneration of Raney catalysts is the subject of some patent documents.
U.S. Pat. No. 6,518,449 B1 discloses a process for the hydrogenation of nitriles with a Raney catalyst in which the spent catalyst, separated from the reaction mixture, is treated with an aqueous alkali solution, in which the anion concentration is at least 0.01 mol/l, at a temperature below 130° C.; the catalyst is then washed with water or an alkali solution, until the pH of the washing water is in the range between 12 and 13.
Patent application US 2010/0267989 A1 describes a HMD production process taking place in a plug-flow reactor, in which a portion of the reacting mixture is continuously withdrawn at the outlet of the reactor and the catalyst contained in said portion is separated from the liquid phase and sent to a regeneration stage, consisting in a first operation of washing with water to remove most of the organic compounds, then a treatment with an inorganic base in order to remove aluminates, and finally a washing step with water or a alkali metal hydroxide solution.
The methods described in these documents suffer however from the drawback of requiring the use of relatively high volumes of basic solutions needing to be disposed of in a safe manner, which generally involves lengthy and energy-intensive pre-conditioning treatments.